can't tell if troll, but, you forget that we have multiple immune mechanisms.
the people who "rarely get sick" are still typically exposed to the same environmental factors as everyone else, including pathogens.
What makes them different is that their primary immune system is able to nip the invader in the bud, usually without the person even realizing that they were exposed to a pathogen. From that point forth they develop secondary immunity to the pathogen, avoiding all the inconvenience of fever and other symptoms.
The people who get sick frequently don't an adequate primary layer of defense for that particular pathogen, and must suffer through the aches and pains and fever until their secondary response can kick in and deal with the invader and give them more permanent protections.
These people get legitimately sick every time they encounter a new variation of a pathogen - typically once every flu season.